Variable frequency drive (VFD) inverters produce pulse width modulated (PWM) high voltage waveforms delineated over power leads to control variable frequency motors in order to control torque, speed, position, etc. The motor electrical frequency is modulated at a much higher rate using pulse width modulation techniques. For a typical electrical frequency of 0 to 600 Hz, the typical inverter modulates the output voltage waveform at 5 to 50 times the output frequency, or in the range of 3 kHz to 30 kHz. This modulation frequency goes by different names, but many sources refer to it as a “carrier” frequency. Historically, this carrier frequency has a very high dV/dt, or edge frequency content, much higher than the motor's electrical frequency or the inverter's carrier frequency. If this edge rate is reduced, then the inverter will dissipate more wasted energy in the form of a temperature rise so the edge rate is usually as high as it can be tolerated in order to keep the wasted energy to a minimum.
Unfortunately, high edge rates in the carrier waveform produce harmful, damaging, or otherwise objectionable levels of Electro-Magnetic Interference, or EMI. Sometimes, this is also referred to as Radio Frequency Interference, or RFI, when the edge rate produces disturbances in the frequency band occupied by commercial, military, or private radio frequency bands.
The RFI/EMI is the result of a parasitic voltage generator constituted by the parasitic capacitance between the power transistor and the heat sink in the VFD, for example. There is also a parasitic load formed by the wire insulation dielectric and the motor cage (motor ground), for example. In one approach to this problem a common mode or current compensated choke is used to magnetically couple the two or more phase windings in the power leads between the VFD and motor. If the currents in the phase windings are balanced and cancel, the choke offers no impedance. But if a parasitic current occurs, unbalancing that cancellation, the choke presents an impedance and suppresses at least a portion of the parasitic current flow in the primary or leakage galvanic paths between the motor and VFD such as through a vehicle chassis frame or concrete floor which gives rise to the RFI/EMI. One approach to this problem is to surround the power leads with a metallic shield such as a tinned copper braid, a solid steel or aluminum electrical conduit, or some other electrical shielding method. As long as this shield does not conduct any current, it will not produce an electric or magnetic field. This need to have no current in the shield is rarely or never met for physical reasons and thus engineers focus on simply reducing the currents to an acceptable level.
However, the shield itself is a good conductor of the parasitic current and becomes a radiator of the undesirable RFI/EMI. This is so in part because the structure of the shield generally has a large surface area and is a good conductor of higher frequencies. One solution to this new problem was to add a secondary internal shield about the power leads but within the primary shield. The inner secondary shield conducts a portion of the parasitic current and the resulting RFI/EMI is shielded by the output primary shield but the outer primary shield still acts as a radiator of the remaining portion of the unwanted RFI/EMI. In addition, shields are expensive and a second one significantly increases the cost.
More recently an improvement in the core material for the choke, nanocrystalline amorphous material, has made the suppression of the parasitic current more critical. For example, that material comes in different forms. One producer, Hitachi Metals, produces two forms. One form, 3 KM, has a high permeability of 70K Gauss/Oersted (CGS) but a lower saturation level and another form, 3 KL, has a lower permeability of 55K Gauss/Oersted but a higher saturation level. This makes the parasitic current a further problem.
While it would be desirable to use the 3 KM, 70K Gauss/Oersted permeability material, there is not much margin for safety in the saturation level and an unbalanced condition might well occur putting the choke into saturation and rendering it useless as a common mode choke. Using the other material, 3 KL, of 55K Gauss/Oersted permeability gains the safety of a higher saturation level but the permeability is lower than the 3 KM material and costs more.